The present invention relates to convergence distortion in color cathode ray tubes, and more particularly, to a method and system for providing two-dimensional color convergence correction.
The primary component of a conventional television set or a computer monitor is a cathode ray tube (CRT) that displays images from an incoming video signal. A color CRT includes a display screen comprising lines of red (R), green (G), and blue (B) phosphors or pixels, and three electron guns corresponding to R, G, and B. The video signal includes a stream of video scan lines comprising R, G and B color signals that are to be displayed on corresponding scan lines and pixel locations of the display screen. The electron guns produce R, G and B electron beams that are deflected onto the screen using electromagnetic circuits such that scan lines are consecutively scanned from the top of the CRT to the bottom during vertical scanning intervals.
A white spot is created on the screen when the R, G and B electron beams coincide on the same pixel at the same intensity. Due to the geometry of the CRT, however, the electron beams travel a longer distance at the corners of the screen than at the center. The disparity in travel distance is even greater in the increasingly popular 16:9 aspect ratio televisions. Due to magnetic lensing errors and the disparity in the electron beam travel distance, the R, G and B electron beams misalign, causing convergence distortions on the CRT.
FIGS. 1A-1C are diagrams illustrating a typical color CRT convergence distortion, where the blue and red electron beams are displaced horizontally and/or vertically from the green electron beam. The goal of convergence correction is to reduce the distortion so that the red and blue electron beams more closely coincide on the same pixel as the green electron beam, as shown in FIGS. 1B and 1C.
Typical methods for convergence correction include the use of static magnets and dynamic electromagnetics. Dynamic electromagnetics apply better control on the corners of CRTs than static magnets, but dynamic electromagnetics are primarily useful only for delta beam configuration convergence correction. In addition, the use of dynamic electromagnetics adds considerably to the cost of the CRT and are therefore not suitable for use in low-end, inexpensive television sets.
Accordingly, what is needed is an improved method and system for correcting convergence distortion. The method and system should be affordable enough so that distortion correction may be applied in both high-end as well as low-end television sets. The method and system should also correct distortion in two dimensions, rather than one. The present invention addresses such needs.
The present invention provides a method and system for providing two-dimensional convergence correction for a color cathode ray tube (CRT). The CRT displays an input video signal comprising a stream of video scan lines which include red, green and blue color signals that are to be displayed on corresponding scan lines and pixel locations of a display screen. The method and system include storing a plurality of scan lines from the video signal into a buffer. For a particular red, green and blue color signal, horizontal and vertical distortion of the red and blue pixels in relation to a position of the green pixel on the display screen are measured. Then, at least one math function is generated that mathematically represents the distortions. Finally, the math functions are applied to control circuitry to control timing of red and blue pixels read from the frame buffer so that output of the red and blue pixels are coincident with output of the green pixel on the display screen.
According to the present invention, two-dimensional convergence correction is provided that effectively aligns the three colors so that all three electron beams are coincident on the display screen to form a nondistorted pixel. The present invention also takes advantages of devices that are equipped with a pre-existing frame buffers to access the scan lines on which misaligned subpixels may fall. Thus, the convergence correction does not add significantly to the cost of the device. Once a pair of red and blue misaligned subpixels are found, the output of the subpixels are timed so that their positions are mathematically shifted left/right and/or up/down as necessary to appear on the same scan line and position of a corresponding green subpixel.